1. Field of the Invention
The present invention relates to a zoom optical system composed of plural lens groups and adapted for performing zooming by varying the distances between the respective lens groups in an optical axis direction, as well as to an imaging lens device with the zoom optical system, and a digital apparatus loaded with the imaging lens device.
2. Description of the Related Art
In recent years, with an explosive spread of a mobile phone and a personal digital assistant (PDA), compact digital still camera units or digital video units have been built in most of these apparatuses. In the apparatuses incorporated with the digital still camera units or the digital video units, a compact image sensor with a small number of pixels, as compared with an image sensor equipped in a digital still camera dedicatedly used for photographing, and an imaging lens device equipped with a mono-focal optical system constituted of one to three plastic lens elements are generally used, considering severe constraints in size and cost required in such apparatuses.
Since the magnification of the mono-focal optical system is substantially in the same level as a human eye, an object to be photographed is required to be set as close as possible to the photographer. In current days when use of an image sensor with a large number of pixels and high performance has been progressed drastically, there is a demand for a compact zoom optical system that is compatible with an image sensor with a large number of pixels, enables a photographer to photograph an object sufficiently away therefrom, and is loadable in a mobile phone or a like apparatus.
As an example of a compact zoom optical system, Japanese Unexamined Patent Publication No. 2002-196240 (counterpart U.S. Pat. No. 6,646,815B2 hereinafter, called as “D1”) proposes a zoom optical system of a so-called negative-positive-positive three-unit type comprising a first lens group having a negative optical power, a second lens group having a positive optical power, and a third lens group having a positive optical power in this order from the object side. In the technique disclosed in D1, miniaturization of the optical system is accomplished, as compared with a conventional optical system, by defining the optical power of the second lens group.
Japanese Unexamined Patent Publication No. 2004-102211 (counterpart U.S. patent application Publication No. US2003/0210471A1, hereinafter called as “D2”) discloses a zoom optical system of a negative-positive two-unit type or of a negative-positive-positive three-unit type, wherein the layout of a lens group arrangement is simplified by defining the configuration of the second lens group to reduce the thickness of the optical system when a lens barrel is collapsed.
It is preferable to use an aspherical lens element to obtain a superfine image. For instance, Japanese Unexamined Patent Publication No. 2002-365543 (counterpart U.S. Pat. No. 6,888,683B2, hereinafter called as “D3”) discloses a zoom optical system of a negative-positive-positive-negative four-unit type, wherein productivity of an aspherical negative lens element in the first lens group is enhanced by appropriately selecting a glass material.
The zoom optical system disclosed in D1 uses four lens elements in the second lens group, which is moved with a largest moving distance in zooming. Accordingly, a load to a driving section is large, and it is difficult to miniaturize the driving section. Further miniaturization is necessary in order to load the zoom optical system in a personal digital assistant (PDA). However, the first lens group of the optical system disclosed in D1 has a large error sensitivity due to decentering, which makes it difficult to produce a compact optical system.
The second lens group of the zoom optical system disclosed in D2 has a weak optical power with a large moving distance, in addition to use of such a large number of lens elements as seven. Accordingly, it is difficult to miniaturize the optical system, and to load the optical system in a PDA. Furthermore, a large impact resistance is required for the PDA, which makes it difficult to make the lens barrel collapsible.
The zoom optical system disclosed in D3 has a long entire optical length in a used state, which makes it difficult to load the optical system in a PDA. It is necessary to reduce the moving distances of the respective lens groups in zooming to produce a compact optical system loadable in a PDA without collapsing the lens barrel.
Furthermore, generally, as a demand for miniaturization is increased, it is difficult to produce lens elements each satisfying a required plane precision, curvature or the like. In addition, it becomes difficult to secure an allowable positional precision in assembling.